This invention relates to a cutting machine for paper, cardboard, or the like and is provided with a hydraulic drive having a power cylinder which is coupled to the movable knife carrier bar by means of a bell crank lever.
In known cutting machines the dead center of the knife carrier is dependent from the end position of the hydraulic piston and thus depends on the accuracy of switching in the hydraulic system or on the position of abutments. During the cutting operation, the knife has to penetrate a few tenths of a millimeter into the backup support for the material to be cut to ensure, for example, that the lowermost sheet of a paper stack is fully severed throughout its entire width. A continuous or occasional deeper penetration of the knife into the backup support, however, leads to a rapid destruction of the backup support and to a premature loss of the sharpness of the knife. Both occurrences adversely affect the quality of cutting. In order to maintain the depth of penetration of the knife independent from the accuracy of the switching operation, in the known hydraulic cutting machines there is provided a fixed abutment for determining the lower limit position of the knife. The switching operation in such a case is effected automatically by means of the pressure increase in the hydraulic conduit after the abutment has been engaged. It is a disadvantage of this arrangement that the fixed abutment is impacted by the knife carrier at the full hydraulic pressure usually with a well audible blow. The sudden pressure increase in the hydraulic system upon engagement of the knife bar with the abutment or the collision of the piston with the cylinder base involves difficulties concerning the sealing arrangements for the hydraulic system. Further, the forceful impacts cause a high stress and a relatively substantial wear in the joints of the mechanical force-transmitting members between the piston and the knife bar.